A valve-operating system for an internal combustion engine designed so that the opening lift amount of an engine valve can be changed continuously is already known, for example, from Japanese Patent Application Laid-open No. 8-74534 or the like. In this valve-operating system, a rocker arm has at one end a valve abutment abutting against an engine valve; one end of a pushrod is fitted over the other end of the rocker arm, and a link mechanism is mounted between the other end of the pushrod and a valve-operating cam.
In the conventional valve-operating system, it is necessary to ensure relatively large spaces for disposition of the link mechanism and the pushrod between the valve-operating cam and the rocker arm, resulting in an increase in size of the valve-operating system. Moreover, a driving force from the valve-operating cam is transmitted through the link mechanism and the pushrod to the rocker arm and hence, it is difficult to mention that the followability of the rocker arm to the valve-operating cam, i.e., the opening/closing followability of the engine valve is excellent.
The present invention has been accomplished with such circumstances in view, and it is an object of the present invention to provide a valve-operating system for an internal combustion engine, wherein the opening lift amount of the engine valve can be changed continuously, while ensuring providing the compactness and ensuring the excellent followability to the valve-operating cam.